CODEWITHSUNDEEP
scalaimplicit
snessarb
snessarb

Reputation: 85

Scala - save creation time on overrided object creation

I want to save the time when I instantiate my object.

With the following code, time changes each time I want to know the creationDate:

class MyClass()

class DatedClass(myClass: MyClass) {
  val creationDate: Long = System.currentTimeMillis()
}

implicit def CachedWrapperClassifier(myClass: MyClass) = {
  new DatedClass(myClass)
}

val obj = new MyClass
println(obj.creationDate)  
Thread.sleep(1000)
println(obj.creationDate)

Both given dates are NOT equal ... any idea why!?

Thank you in advance!

UPDATE:

First, thank you for your responses!

I want to choose the Trait solution but this does not work anymore, if I want to use a separated method to retrieve the object ...

trait CreationTime {
  val creationTime: Long = System.currentTimeMillis()
}

class MyClass()

def getMyClass: MyClass = {
  new MyClass with CreationTime
}

val obj = getMyClass
println(obj.creationTime) // creationTime is not accessible
Thread.sleep(1000)
println(obj.creationTime)

But I can NOT touch "MyClass".

Any idea how to solve this?

Upvotes: 1

Views: 71

Answers (4)

Mike Cialowicz
Mike Cialowicz

Reputation: 10020

I think you probably want to use a trait, which will simplify your code:

trait CreateTime {
    val createTime: Long = System.currentTimeMillis()
}

You can attach it to your class either by extending it in the class definition:

class MyClass() extends CreateTime

Or when you create your object:

val obj = new MyClass with CreateTime

Also, pay attention to your variable names. What you called creationDate really is not a date... it's a time (or a timestamp). It's important to be clear with your variable names.

Upvotes: 2

Jesper
Jesper

Reputation: 206796

One way to do this differently is by using a trait as a mixin:

trait CreationDate {
  val creationDate: Long = System.currentTimeMillis()
}

val obj = new MyClass with CreationDate
println(obj.creationDate)  
Thread.sleep(1000)
println(obj.creationDate)

Upvotes: 3

David Holbrook
David Holbrook

Reputation: 1617

obj is an implicit conversion wrapper, so you are getting a new instance each time you use it.

if you add a println you will see what is happening

class DatedClass(myClass: MyClass) {
  println("new instance")
  val creationDate: Long = System.currentTimeMillis()
}

Upvotes: 1

tmbo
tmbo

Reputation: 1317

Your compiler will replace your calls to creationDate with the implicit conversion. To call the method, the compiler will create a new wrapper object for each call to creationDate. What the compiler does has the same effect as doing the following:

val obj = new MyClass
println(new DatedClass(obj).creationDate)  
Thread.sleep(1000)
println(new DatedClass(obj).creationDate)

Since the constructor is called twice, the creation dates are different.

Upvotes: 2

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